In general, this invention relates to the same field as the following U.S. Pat. Nos., which are herein incorporated by reference: 4,007,454, 4,165,505, 4,477,860, 4,606,008, 4,429,308, 4,433,332, 3,500,365, 3,845,377, 3,732,553, 3,068,456, 3,222,591, and 3,355,806. Application of the teachings of some of the aforementioned patents to practical manufacturing techniques is basically shown as element 1 in FIG. 1. In this Figure, PC board 2 is shown with glass insert 6 plus electronic circuits 4 deposited on its upper surface. Glass insert 6 represents a glass substrate onto which its under surface electrodes 5 (indium tin oxide for example) and center electrode 12 are deposited, in a manner taught by the above cited prior art. The glass is thought by the prior art to be necessary so that the meter hands 9 and dials 8, disposed beneath glass substrate 6, can be visually observed. Substrate 7 is spaced apart from substrate 2 by means of clips 3, which are described in U.S. Pat. No. 4,556,844, the contents which are incorporated by reference.
Substrate 7 schematically represents the face of a multi-dial meter on which there may be a plurality of dial faces 8. Only one dial face 8 is shown, but in practice, a plurality is contemplated. Each dial face has a rotatable member, hand 9 plus axle 19. The prior art teaches that dial hand 9 and axle -9 rotates in a rotating electric field created by electrodes 5. Elements 4 represent the electronic circuitry connected (not shown) to electrodes 5 and center electrode 12 necessary to detect, collect and transmit data caused by the presence of hand 9 in the rotating electric field created by electrodes 5 and a polyphase voltage continuously applied to electrodes 5, all as described by the prior art.
Glass plate 6 is affixed to printed circuit board by means of a conductive epoxy 10. After the composite of substrate 2 and glass plate 6 is made, the surface on which electrodes 5 and 12 are disposed should be coated with a plastic, otherwise there will be signal distortions believed to arise out of leakage between electrodes 5 and a center electrode 12 caused by water vapor at a relative humidity of 55% or more. Signal distortions can be either (i) erroneous reading, i.e., the meter reading device knows that its reading is bad and so indicates; or, (ii) a misreading, namely, a wrong number altogether, without acknowledgment by the device that the number is wrong. In order to protect against such distortion, the printed circuit board and glass substrate must be first washed with a solvent (trichlorethylene, for example) and then the surface coated by using a conformal sealer, such as Paraylene resin made by Union Carbide. It is essential that the solvent first must be used. If not, the coating will not properly adhere. However, when the solvent is used, the conductive epoxy 10 is dissolved or weakened, resulting in poor quality. If the solvent is not used and the printed circuit board and glass substrate are coated nonetheless, uneven adhesion of the coating results, yielding signal distortions. Applicants have found that with uncoated prior art devices, the higher the relative humidity the more the distortion, i.e., distortion appears to be proportional to the relative humidity. Coating is not necessarily believed to be a complete or a permanent solution itself because there are present indications that it merely slows the distortion process.
The prior art, exemplified by the above disclosed patents, employs a large (one megaohm) resistor 58 (FIG. 15) disposed on a printed circuit board (PCB) to cause a voltage drop. Small currents 57 are involved, thus a large resistor 58 is required to produce a usable signal. The glass substrate 6, upon which the excitable electrodes of the prior art are deposited, operates as a capacitor in combination with the electrodes 5 and 12. Glass capacitor 61, in combination with the above-mentioned large resistor 58, forms an RC circuit, a high impedance point at the juncture of the glass and the large resistor 59. In practice, this usually takes place at the interface of the glass and the PCB itself. A high impedance point and humidity do not mix. Signal distortion can result. Getting rid of the prior art glass plate 6, on which electrodes are deposited, is a first step towards the solution to the humidity problem and ease of manufacturing. Otherwise, the manufacturing problem of the glass plate first cleaned with a solvent and then a conformal coating placed thereover remains unsolved. A second step towards solution to the humidity problem is the employment of a low input impedance receiver circuit, such as that shown in FIG. 8, which minimizes variations due to humidity A third step is to maximize the signal contribution, due to the dial hand being in local proximity of the electrical field with respect to the signal size when the dial hand is not in local proximity thereto. Compare FIG. 5 with FIG. 6.
FIG. 15 is a simplified schematic description of prior art RC circuit created by the glass plate 6 acting as a capacitor (element 61) with resistor 58, (a one megaohm plus resistor) to create a necessary voltage drop so that a proper signal could be detected and transmitted by field effect transistor 56. The capacitance effect of element 61 arises out of center electrode 12, excitable electrodes 5 deposited on glass substrate 6 and electronic circuit 4 plus a polyphase voltage continuously supplied to electrodes 5 as taught by the prior art.
It would be desirable to have a meter reading device that would have no signal distortions at high humidity (55% RH and greater). Specifically, a desired meter reading device of this type would be fully operable and give substantially no distorted readings between 5 and 95% relative humidity (non-condensing) for an indefinite time between -40.degree. and +70.degree. C. Applicants have found a way to solve this relative humidity problem: placing excitable electrodes 11-1 through 11-10 (FIG. 14) and center electrode 12 between the dial face 8 and rotatable member (meterhand 9). See FIG. 16.
Such a structure, along with receiver circuit 24 as shown in FIG. 8, is used to measure the amplitude of a plurality of fields, one at a time, storing the measurements until all fields are read and then comparing such measurements to ascertain the location of a meterhand. In contrast, the prior art measures the resultant of a plurality of fields arising out of a polyphase voltage continuously applied to an array of excitable electrodes, stores and compares these resultants to certain preset values, to determine a voltage phase shift
By following the teachings of this invention (1) there results a stronger signal to baseline ratio than achieved by the prior art, the baseline being the values read with the dial hand not in close proximity and the signal being the value read with the dial hand in close proximity to an excited electrode center electrode plate-like capacitor; (2) glass-electrode composite requirement is removed, which results in plastic (conformal) coating manufacturing simplicity, and (3) the receiver circuit of the invention employs a lower impedance than that taught by the prior art.
A device employing this invention has been observed to operate between 5 and 95% relative humidity for an indefinite time between -40.degree. and +70.degree. C. with substantially no distortion. A sealant and/or electronic guard can be used as insurance against signal distortion arising out of high humidity.
One structural difference between the invention and that of the previously identified prior art is that the placement of the guard, the excitable and center electrodes affixed to plate 16 in the space partially delimited by the circumferential path described by rotating dial hand 9 (rotatable member) and dial face 8. See elements 11-1 through 11-10, 12, and 21 of FIGS. 14 and 16. Another difference is that the center electrode 12 senses current amplitude, not a voltage change (shift in voltage phase), as does the prior art. When hand 9 is disposed over a given excited electrode, capacitance coupling is increased between the given electrode (any one of electrodes 11-1 through 11-10) and center electrode 12. This results in a greater current emitted by center electrode 12 than if the hand were absent. Since the position of a given excitable electrode is known, the presence of this "greater current" then identifies the position of the hand. Another difference is the elimination of a glass substrate in combination with excitable electrodes.
The prior art technique has drawbacks: (1) the glass-PCB-dial hand alignment is very critical and difficult to hold, (2) the glass has a propensity to contain large amounts of sodium, which at high humidities is very conductive. Soda lime silica glass is inexpensive thus it has a tendency to be used whereas specialty glasses that are devoid of, or low in, sodium are expensive and are not used, (3) the electrode pattern etching on the glass is expensive, (4) the epoxies used do not hold up to cleaning agents commonly used and without proper cleaning, the conformal coating will not adhere properly. The instant invention has the electrode pattern etched in the printed circuit board (PCB) along with the circuit interconnects using a standard process. Dial labeling information is reproduced on the PCB using standard soldermask and silkscreen techniques. Mounting of the PCB is below the dial hands and above the dial face itself. This new positioning of the PCB does the most to resolve the problems, which can occur at high humidity. Since the dial hands can be seen, it allows for the elimination of the glass and epoxy, which in turn allows for proper cleaning of the PCB. This in turn will promote good adhesion of the conformal coat, which will reduce the effects of humidity. The elimination of the glass and epoxy simplify the manufacturing process.
The prior art also uses a technique of forming a swirl pattern for the excitable electrodes (see U.S. Pat. No. 4,606,008 for example) so that they interlaced to promote the influence of the dial hand on the multiple excitable electrodes in combination with the center electrode. All of the excitable electrodes are continuously excited at the same time with a polyphase voltage to form a rotating electrical field. The resultant of this electrical field on the center electrode is measured. When the dial hand is introduced into the rotating electrical field, it causes a phase shift in the measured voltage as compared with no dial hand present, this being called a baseline and the measured value with dial hand present is a signal. The measurement is made by a high input impedance receiver circuit and the electrical field causes a voltage to be developed at the high input impedance.
The prior art technique described requires fine matching of the components involved. Also, measuring the resultant field requires initial calibration of the rotating electrical field, which is done with great difficulty by gluing a conductive "tweak dot" on the non-electrode side of the glass after adjusting the tweak dot so that it causes an acceptable measurement value with no dial hand present. Also, the signal to baseline ratio is very small because the dial hand influences only a small portion of the total electric field, which makes it difficult for easy measurement. The high input impedance of the receiver is also susceptible to influence by small leakage currents, appearing mostly in high humidity environments.